The stock market has been tanking since well before the election.
It's been doing so because the rich are pulling all their money out of it. They're doing that because of the chaos that ensued, following the housing crisis.
The housing crisis was the result of rich people throwing all their money into an irrational housing bubble, which collapsed on them. It was a classic example of the free, unregulated market screwing itself into a tailspin, thanks the profound stupidity of large numbers of rich people. This sort of thing has been happening for hundreds of years. You've heard of the Dutch Tulip Madness?
The price of oil has also been diving very fast. This is because all the stupid rich speculators who had driven it so unrealistically high before, have pulled all their money out of there as well. It was yet another bubble, an artifact of an unregulated market, created by stupid rich people.
All of this is the direct result of good old Reaganomics, which holds that the answer to all our problems lies in letting rich people accumulate as much money as they possibly can, and charging them as little in possible in taxes for the privilege of exploiting our economy in any way they can imagine.
Thanks to that stupidity, rich people have accumulated a greatly inordinate fraction of the total money supply. And thus they have an inordinate degree of control over the economy.
But, as should have been obvious from the beginning, they have consistently demonstrated that they are not invariably going to produce the kind of economic wizardry, with that power, that Reaganomics would have you believe they would.
Much the contrary. Turns out, they're quite often liable to act like hysterical, stupid, and/or totally crooked and dishonest and irresponsible people, with absolutely no concern the nation as a whole, and with whom the public has no reason to place it's trust, at all.
And what you are seeing here is the fruits of their labors.